This invention relates to semiconductor subscriber loop interface circuits (SLIC""s) and, more specifically, the invention relates to programmable subscriber loop interface circuits.
Telecommunications systems employ access products (e.g., subscriber interface units) for interfacing with each individual subscriber unit. An access product may include a SLIC for performing two to four wire conversion, battery feed, line supervision, and common mode rejection.
In a subscriber loop, the subscriber unit is generally powered from the SLIC through interface lines. Typically, subscriber loop interface lines are connected to the subscriber terminal pair of a SLIC. SLIC""s are generally powered with a xe2x88x9248 V power supply and are backed up with a power supply of the same magnitude. Typically, the SLIC must deliver 3 to 5 volts DC to a subscriber unit (e.g., a telephone) for off-hook states and deliver xe2x88x9242.5 to xe2x88x9256 volts DC (i.e., open circuit voltage levels) for on-hook states.
In a voltage feed, current sensing, subscriber loop interface circuit, current in the loop is sensed and in response to the sensed current; voltage being applied to the subscriber terminals is varied. Sensed current may be referred to as loop current. Voltage applied across the subscriber terminals in response to loop current may be known as loop voltage. Typically, the range in values and the relationship between loop current and loop voltage is set internal to a semiconductor SLIC chip when the SLIC is manufactured.
Overhead voltage which may also be referred to as overload voltage are a range of voltage available for sudden voltage fluctuations. Typically, overhead is the voltage range between a saturation voltage and the loop voltage.
A problem in typical semiconductor SLIC chips may be the inability to provide adequate off-hook overhead for long loop length applications without needing a separate loop current to loop voltage relationship for off-hook and oh-hook conditions.
In typical prior art SLIC""s, on-hook and off-hook conditions have separate states (i.e., separate loop current to loop voltage relationships). With reference to FIG. 1, over the range of loop: current Iloop, the on-hook loop voltage 12 is greater than the off-hook loop voltage 10. The off-hook loop voltage 10 decreases linearly over the range of loop current while the on-hook loop voltage 12 is constant for a period and then decreases linearly.
Linear decrease of loop voltage in relation to increasing loop current may be referred to as resistive feed. A constant loop current in relation to variation in loop voltage may be referred to as constant current feed. Typically, prior art SLIC""s only provide either constant current feed or resistive feed.
In one SLIC design having constant current feed, the SLIC includes circuitry to prevent the voltage on the subscriber terminals from increasing above a saturation voltage. As the current drops in the line, the voltage transitions from the overload level for the off-hook state to the overload level for the on-hook state. In such a SLIC design, the saturation voltage may be set internally or externally to define overhead. The saturation voltage needs to be set so that overhead voltage on long loop application may be adequate for expected signal conditions. The saturation voltage level is typically set independent of the loop current limit setting ILimit by measuring loop voltage and comparing loop voltage to an on-chip reference value. The saturation voltage level may also be set by an internal or external resistor. Setting the saturation voltage closer to Vbat decreases the on-hook signal capability of the SLIC. But increasing the on-hook signal capability may decrease the ability to achieve minimum on-hook open circuit requirements for some types of terminal equipment. In addition, setting the saturation voltage closer to Vbat increase SLIC capability in supplying power to long loops.
With reference to FIG. 2, in one prior art technique, an increasing transition 20 occurs between an on-hook state 24 and an off-hook state 22. The transition occurs at a current Ish that may represent a switch hook detect threshold. In the off-hook state 22, loop voltage is generally adequate to power long loop length applications. In the on-hook state 24, voltage may be set during manufacturing to be farther away from Vbat thus providing a broader range available for setting saturation voltage. A drawback of this technique is that there is instability around the transition because of the abrupt change.
With reference to FIG. 3, in another prior art technique, the TIP to ground voltage is kept substantially constant for loop current up to 6 mA, generally estimated to be the maximum line leakage in most systems. When the 6 mA current is exceeded, the overhead voltage magnitude rapidly increases to support speech and or teletax signal levels (see the vertical voltage decrease at the 6 mA threshold). In this technique, a virtual battery Vvirbat is used to define a virtual reference for resistive feed 31. Saturation voltage is not used because the off-hook voltage state of the SLIC are defined from the virtual battery and not from the loop voltage. Loop voltage in off-hook state is referenced to the virtual battery which increases the SLIC""s ability to power long loops. In this technique, overhead voltage in the on-hook state is fixed on-chip and is not user programmable. The virtual battery and associated resistive feed slope may be set by one resistor. Another resistor may be required to set current limit. A drawback of this technique is that the switch detect threshold may not be set below 6 mA. Another drawback is that to increase the size of off-hook overhead, the SLIC must use resistive feed or use an external resistor Roh to offset the virtual battery curve. A further drawback is that if constant current feed is used, the overhead of the manufactured SLIC may not be increased, except by using the external resistor Roh. Even external resistor Roh is used, on-hook overhead is not user programmable.
Accordingly, it is an object of the present invention to provide a novel SLIC that overcomes the drawbacks of the known prior art.
It is another object of the present invention to provide a novel method of powering a subscriber loop.
It is yet another object of the present invention to provide a novel SLIC having user programmable parameters allowing a user to program the SLIC to meet varying requirements.
It is still another object of the present invention to provide a novel voltage feed, current sense SLIC which provides adequate overhead voltage without relying on the digital transition signal between separate on-hook and off-hook states.
It is a further object of the present invention to provide a novel SLIC having means external to SLIC for selecting SLIC operating parameters.
It is yet a further object of the present invention to provide a novel SLIC having means external to the SLIC for programming switch hook detect threshold, current limit, and overhead voltage(s).
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.